The overall goal of this research is to understand the roles of a membrane/soluble glycoprotein complex (Muc4/sialomucin complex, SMC), which is a major contributing mucin at the ocular surface and in the ocular tear film. Muc4/SMC is composed ofa mucin subunit ASGP-1 linked to the plasma membrane via an N-glycosylated transmembrane subunit ASGP-2. The complex is encoded by a single gene, expressed as a 9.2 kb transcript and synthesized as a precursor of approximately 300 kDa. ASGP-1 acts as an anti-recognition factor at cell surfaces. ASGP-2 has two EGF-like domains and can act selectively as a ligand for the receptor tyrosine kinase ErbB2. In addition to the membrane form of Muc4/SMC, corneal epithelia produce a soluble form which is present in tear fluid and loosely associated with the superficial surfaces of the ocular surface epithelia. Our recent studies on transfected cells have shown that Muc4/SMC can act as a repressor of apoptosis and inducer of the cell cycle inhibitor p27kip. Moreover, Muc4/SMC is localized to the most superficial, most differentiated cell layers of the ocular surface epithelia. Our hypothesis is that Muc4/SMC acts to protect ocular surface epithelia by several mechanisms. In recent studies we have established a multilayered cell culture of rat epithelial cells. We will use this culture system to investigate our hypothesis that Muc4/SMC plays an important role in desquamation of cells from the superficial epithelial layer (aim 1). In addition we will develop a transgenic mouse overexpressing and mis-localizing Muc4/SMC in the corneal epithelium to test this hypothesis (aim 1). We will further use a wound healing system and the culture system to investigate the behavior and possible participation of Muc4/SMC in corneal wound healing (aim 2). Finally, we propose to develop a Muc4/SMC knockout mouse for a more comprehensive view of the roles of Muc4/SMC at the ocular surface (aim 3). These combined studies should help to provide us with a more complete picture of the functions of Muc4/SMC at the ocular surface and insights into the roles of this mucin in ocular surface diseases and aberrations.